Ran head hunter on my archers in the last playthrough with great success. Almost every enemy type has units with weak head armor.
Raiders of all types can skip head protection.
Noble houses have billmen, arbalesters and sergeants.
Undead have nachzehrers and wiedergangers.
Orc warriors sometimes dont have helmets in addition to youngs and berserkers.
Against ancient dead archers arent very useful, headhunter or not.
The good thing about having it on archers is that you can target the specific helmetless enemy without having to run across the whole field.